A. Prior Art Methods of Physiological Pressure Monitoring By Liquid-Column Catheters
Clinically, it has heretofore been common practice to utilize liquid-filled tubular pressure monitoring catheters for measuring pressures within anatomical passageways, organs, compartments and cavities of the mammalian body. Typically, such medical pressure monitoring catheters comprise elongate tubular catheters having one or more hollow lumens extending longitudinally therethrough, and one or more openings near the distal end of the catheter through which pressure changes in the surrounding medium are received into the catheter lumen(s). The proximal end of the catheter is typically connected to a physiological pressure transducer and the transducer and catheter lumen(s) are charged with a liquid (e.g., saline solution) through which changes in pressure are transmitted from the distal end of the catheter, to the extracorporeally located pressure transducer. The pressure transducer converts received changes in pressure to electrical signals which may be transmitted to a corresponding display apparatus (e.g., a bedside monitor) or recording apparatus (e.g., magnetic tape, chart recorder).
Liquid filled pressure monitoring catheters of the above-described type have been used to monitor various pressures within the mammalian body, including arterial pressure, central venous pressure, pulmonary artery pressures, pressures within the chambers of the heart, intracranial pressure, intrauterine pressure during labor & delivery etc...
In most physiological pressure monitoring applications using liquid filled catheters, it is necessary to maintain sterility of the liquid-filled catheter, the pressure transmitting liquid and all of the portions of the pressure transducer which come into contact with the pressure transmitting liquid so as to avoid the introduction of pathogenic organisms into the body. Also, in many long term clinical applications, it is necessary to provide a continuous or periodic flow of liquid (e.g., heparinized saline solution) through the catheter to prevent occlusion (e.g., the formation of blood clots) of the catheter lumen and to maintain patency of the catheter. In applications such as arterial pressure monitoring or central venous pressure monitoring, it is common practice to employ a constant flush apparatus such as that commercially available as Model No 42684-05, available from Abbott Laboratories, Chicago, Ill., in conjunction with the pressure monitoring catheter to maintain a slow continuous flow of heparinized saline solution or other liquid through the catheter. Such constant flush apparatus must maintain the flow of liquid at a sufficiently low level as to avoid interference with the pressure monitoring function of the catheter. The utilization of such constant flushing apparatus adds to the overall expense of the pressure monitoring catheter system.
Additionally, during set up of a liquid-filled pressure monitoring catheter of the prior art, it is typically necessary to prime the catheter with a sterile liquid and to carefully remove any air bubbles which form within the catheter or transducer, as the presence of such air bubbles may adversely affect the responsiveness of the transducer. Thereafter, while the catheter remains inserted, any inadvertent introduction of air bubbles during flushing or manipulation of the transducer/catheter may result in damping of dynamic response of the system. Such damping may result in inaccuracy of the monitored pressure, especially in applications wherein a continuous pressure wave form is to be monitored (e.g., arterial pressure monitoring).
Additionally, when using the liquid filled pressure monitoring catheters of the prior art, it is typically necessary to make certain that the pressure transducer is on a level plane with the pressure-receiving aperture(s) of the catheter to avoid inducement of hydrostatic error in the monitored pressure. After the transducer has been leveled with the catheter, any movement or adjustment of the positioning of the patient will result in a hydrostatic error in the pressure reading obtained, unless the pressure transducer is correspondingly relieved.
B. Prior Art Devices Which Measure Physiological Pressure Transmitted Through a Gaseous Medium
The following United States and foreign patents/patent publications have described pressure measuring catheters, and other pressure transmitting systems, wherein a gas is utilized as a pressure-transmitting medium in at least a portion of the systems: U.S. Pat. Nos. 2,840,069 (Squire, et al.), 4,227,420 (Lamadrid), 4,300,571 (Waldbilling), 4,314,480 (Becker), 4,648,406 (Miller), 4,841,984 (Armeniades et al.), 5,105,820 (Moriuchi, et al.), 5,279,308 (DiSabito, et al.); Foreign Patent publications: WO82/02657 (Ebert), WO86/03957 (Spiegelberg), WO90/11717 (Utah Medical Prod., Inc.).
In particular, WO86/03957 (Speigelberg) describes a medical pressure monitoring catheter having a gas-filled lumen extending longitudinally therethrough and in gaseous communication with the inflation space of a partially inflated bag. Changes in pressure against the outer surface of the bag are transmitted through the gas-filled lumen of the catheter to an externally located pressure sensor. An electromechanical pump system is utilized to provide the desired degree of bag inflation to facilitate the intended pressure monitoring function of the catheter.
Although the prior art has included at least one gas-column pressure monitoring catheter and a number of other devices which utilize gas as a pressure transmitting medium, there remains a need in the art for the development of new pressure monitoring catheters which utilize gas, rather than liquid, as the pressure-transmitting medium and which are designed constructed and configured to avoid some or all of the problems, sterility requirements and/or limitations associated with the prior art pressure monitoring catheters.